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Scopes for Airguns, part II: Optics

Shopping for a scope you’ll encounter a dizzying variety of optical choices- fixed, zoom, low power, 6x, 3-9×30, 8×56- how can you make sense of all this? Start by understanding the numbers and what they mean.

The first number- or number- indicate the magnification of a scope. Two numbers seperated by a dash mans that the scope is a zoom type, with variable magnification. The number on the other side of the ‘x’ is the size of the objective lens in millimeters. Thus a 2-7×32 scope has an adjustible magnification of from 2x to 7x- that is, objects look bewtween 2 and 7 times as large as they would when viewed with the naked eye- and the objective lens is 32mm in diameter. Magnification is certainly simple enough, but what does the size of the objective lens mean? How does that affect performance?

Basically, the larger the objective lens, the more light is gathered, and the brighter the image- but only up to a point. If you divide the objective lens diameter by the magnification, you get what’s called the exit pupil size- that is, the diameter of the bundle of light presented to your eye. The bigger the exit pupil, the brighter the image, until it equals the size of your eye’s pupil. Beyond that, there’s no benefit. So how big is you eye’s pupil?

It varies, depending on the brightness of the environment, and a person’s age. In bright sunlight, the pupil might contract to a 1-2mm or less; at night, a young person’s pupil might be anything from 5 to 9mm, with 7mm being around average. As we age, this decreases on the order of a millimeter or more over the course of one’s life.

Most shooting is done in daylight, and so a pupil size of more than 2-4mm is probably unecessary. Shooting around dusk- say, shooting vermin around a farm or garden- you might profitibly go as high as 7mm.

As you can see, the higher the magnification, the larger the objective lens size you need to acheive a given exit pupil. The old Beeman 2×15 SS2 scope looks tiny until you realize that it has a 7.5mm exit pupil; making it any larger would be a waste of glass. The popular 3-9×32 scopes marketed by many companies for airgun use has an exit pupil of only (32/9) = 3.5mm at maximum magnification; fine for daylight use, but too dim for use at dusk. At the other extreme, I saw a 3-9x80mm scope advertised in a British airgun shooting magazine as the ultimate night hunter- 80/9 = 8.88mm, far bigger than 99.99% of the pupils out there in the population.

Most of us need no bigger than a 4mm exit pupil for most uses, as we do most of our shooting in daylight, unlike our British cousins who sometimes hunt at night. But what about magnification? How much do you need? As always, it depends on the ranges you shoot at, and the targets you’re shooting at.

At the high end, Field Target shooters like to get as much magnification as possibkle, for two reasons- they’re shooting at some very small targets, and high magnification makes it easier to use your scope as a range finder. That’s critical in figuring out your trajectory. Most shooters seem to consider 6-18x to be a minimum range, with the use of 8-32x and 8-40x being not uncommon.
So why doesn’t everyone use a 8-40x scope? Cost is one reason, size another (they’re big!) and ease of use a third. It is extremely difficult to acquire a target at 40x- field target shooters typically acquire the target at a lower magnification, and then zoom to maximum and focus to find the range. Then they adjust their elevation and windage, and aquire the target yet again. Fine for shooting a steel target fastened to the ground, but not to good for game.

Hunters typically use lower power scopes, or even fixed scopes. I have a Burris 6x mini on my Theoben Sirocco. It’s small, very rugged, and quick and easy to use. It doens’t allow for the same precision when shooting at 55 yards as does my Simmons 6x18x scope, but I try to hunt at shorter distances than that. (A miss in Field Target means you lose a point; in hunting, it means you wound an animal, and as hunters, we should always try for quick, humane kills.) Scopes in the 2-7x and 3-9x are very popular, and indeed are probably ideal for hunting.
Plinkers, who shoot for the pure fun of it, have the widest range of choices. Some like big target scopes, others like small, simple scopes, and some like the non-magnifying scopes and optical sights commonly seen on competition handguns and combat rifles. These don’t allow the same precision in pellet placement as do magnifying scopes, but a good shooter can do well with them, and they’re the quickest and easiest to use of any sight system.

What about the leaf sights that come on just about every air rifle?  Why not just use them? Well, while they’re more than adequate for plinking, they simply don’t have the accuracy needed for airgun huting, let alone target. Sure, a lot of deer have been taken over the years with iron sights, but the lethal area on a deer- the heart and lungs- is much bigger than entire body of most of the animals airgun hunters pursue. There are always exceptions- I know of some very successful hunters who use an old Sheridan pump gun with the stock sights- but for most of us, a scope is a better choice.

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